UV light returns colour to million-year-old seashells

Three shells in visible light (top), UV light (middle) and with colours reversed (bottom), showing that the strongest glow is associated with the darkest pigmentation.
PLOS One/J. Hendricks

If you've ever collected seashells on the beach, you'll have gathered up all kinds -- some patterned and shiny, some white and dull, stripped of all colour, pattern and gloss by the tumble of waves rolling the shell across the sand.

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As one researcher has discovered, it is possible to reclaim those lost colours -- at least on the shells of cone snails. Associate Professor Jonathan Hendricks of San Jose State University's Department of Geology has been using the technique to examine ancient cone snail shells millions of years old, and has discovered several new species.

"Modern cone snails are famous for their unparalleled diversity in the marine realm (the group includes the genus Conus, which -- with over 600 living species -- is the most diverse genus of marine animal alive today), as well as the fact that they are venomous predators -- some species are even known to have caused human fatalities," Hendricks explained.

"The purpose of my research on fossil cone snails is to increase our understanding of species-level patterns of evolution, extinction and biogeography, with the broader goal of understanding the intrinsic and extrinsic factors responsible for those patterns, and, more generally, observed biodiversity."

How and why cone snail shells are patterned is not fully understood, especially since some cone snails coat their shells in an opaque substance, or lay buried in the mud on the sea floor -- their shells hidden from view. Although there is a strong theory that suggests that the pattern is based on the mollusc's neural activity, researchers still don't know for certain.

However, we do know that there are over 700 species of cone snails still in existence, and shell patterns can vary in fine detail within a species -- although each has a set of broader parameters between which it falls, and closely related species tend to display variations on general themes of patterning.

Cataloguing cone snail shell fossils is a little trickier -- not least because the shells have been stripped of colour. But, as discovered by researcher Axel Olsson in the 1960s, shining UV light on such fossils causes the remaining organic material that caused the patterning to fluouresce -- revealing the pattern, if not the colouration. This, Hendricks adds later in Photoshop.

By applying this technique to 28 different cone shell fossils from three Neogene-associated coral reef deposits in the northern Dominican Republic, aged between 4.8 and 6.6 million years old, Hendricks was able to reveal their patterns. Doing so led to the discovery of 13 new species among them a particularly striking polka-dotted shell, now extinct.

"Each of the three reef deposits contain a minimum of 14-16 cone snail species, levels of diversity that are similar to modern Indo-Pacific reef systems," Hendricks wrote.

It also revealed similarities to the shell patterning of some modern Caribbean species, indicating that some lineages have survived millions of years in the same region.

The research, however, leaves open another mystery: why the shells glow.

"While the use of UV light to reveal ancient shell colouration patterns has proved to be a useful technique for understanding the systematics of some fossil molluscs, we still do not have a clear understanding of exactly what compounds are responsible for pigmentation in modern shells, much less what matter is actually fluorescing in the fossil shells," Hendricks wrote.